Many applications require varying the light intensity of a semiconductor light source such as a light-emitting diode (LED). For instance displays based on LED back-lighting technology require many LED zones to achieve a high contrast ratio between bright and dark images. Each LED zone also referred to as LED channel can be used to achieve local dimming and create realistic images.
Dimming may be achieved using a pulse width modulated PWM signal to drive a dimming transistor. The pulse width modulated signal is repeatedly switched between high and low states. Upon receiving a high state at its gate, the dimming transistor couples the light source to a current source, and the light source emits light. Upon receiving a low state at its gate, the dimming transistor decouples the light source from the current source and the light source stops emitting light. Therefore the dimming effect is achieved by enabling and disabling of the dimming transistor repeatedly at a sufficiently high frequency, while the level of dimming is changed by changing the duty cycle of the PWM signal.
Traditionally, the frequency of the PWM signal ranges between 50 Hz and 960 HZ. However, better dimming performances can be achieved at higher frequencies. For instance above 20 KHz audio noise can be greatly reduced or eliminated. At such high frequencies the turn-on delay of the PWM signal affects the dimming accuracy and linearity. Existing solutions based on the measurement of the turn-on delay and subsequent delay compensation lack reliability especially when the PWM signal is set with a high duty cycle.